Fluid powered motors driving an additive injection pump connected to a source of fluid additives are typically installed in a line containing primary fluid. Operation of the fluid motor reciprocates a piston within a cylinder of the additive injection pump to draw a quantity of secondary fluid into the primary fluid with each reciprocation. Such devices have been applied to add medication to drinking water for poultry and livestock, treat water with additives, add fertilizer concentrate to irrigation water, or add lubricant or cleaning agents to water. In liquid additive injection pumps, such as that shown in U.S. Pat. No. 4,809,731, piston ring seals have been used to engage the bore wall of the cylinder as the piston is reciprocated therein.